This invention relates to preventing an analog color video signal from being satisfactorily copied and, more particularly, to a technique for modifying the usual color burst interval of the analog color video signal such that when the modified video signal is recorded and then reproduced, the reproduced color video picture is severely deteriorated.
Copy protection techniques for preventing the unauthorized copying, or re-recording, of analog video signals are well known. Such techniques, also known as copy guard processing, generally employ either or both of two separate processes. In one copy guard technique, the usual automatic gain control (AGC) circuit of an analog video recorder is deceived into detecting a signal level that appears to be too high, thereby reducing the gain of the video signal that is recorded; and as a result, the level of the recorded signal is too low to recover. Such an AGC copy guard process inserts pulses, referred to as pseudo-sync pulses, in predetermined line intervals of the usual vertical blanking interval of the analog video signal.
FIG. 10A schematically represents those line intervals of the vertical blanking interval into which the pseudo-sync pulses are inserted; and FIG. 10B represents a number of cycles (e.g. 5 cycles) of such pseudo-sync pulses.
Typically, the AGC circuit of a consumer-type analog video recorder detects the difference between the sync tip level and the reference level of the video signal in the vertical blanking interval. This difference is known as the AGC reference level, shown in FIG. 10C, and differences in the AGC reference level are used to vary the gain of the recording circuitry. However, when the pseudo-sync pulses are inserted into these line intervals, such as shown in FIG. 10B, the AGC reference level detected by the AGC circuit of the video recorder now extends between the negative tip of the pseudo-sync pulses and the positive tip of those pulses, the latter admitting of a level p above the expected reference level. This deceptively large AGC reference level causes the AGC circuit of the video recorder to reduce the gain of the recording circuit, thereby reducing the recorded level of the video signal to substantially zero.
While the use of such pseudo-sync pulses has proven effective in most consumer video recorders, a number of video recorders do not rely upon the difference between the sync tip and the reference level of the video signal in the horizontal blanking interval to control the gain of the recording circuits. Examples of such video recorders include .beta.-type recorders, 8 mm video recorders and certain sophisticated VHS type video recorders.
In an effort to prevent the unauthorized copying of color video signals in such analog video recorders, a so-called color stripe copy guard technique has been introduced. In the color stripe process, the phase of the usual color burst signal is inverted on a generally repetitive basis. For example, the color burst signal in a block of two line intervals or in a block of four line intervals is inverted; and each frame is formed of a number of blocks having a repetitive pitch, for example, twenty lines. As a numerical example, the phase of the color burst signal may be inverted in lines 22 and 23, 42 and 43, 62 and 63, etc. Because of such phase inversions, when this analog video signal is recorded, the automatic phase control (APC) circuit of the recording circuit is subjected to error; and the resultant video picture that ultimately is reproduced from that recorded signal exhibits annoying color stripes, such as shown in FIG. 11.
Since the color burst signal of a relatively small percentage of the line intervals exhibits phase inversion, the phase locked loop (PLL) circuit of the APC circuit in a conventional television receiver normally is not affected. This is because the time constant of such PLL circuit, and particularly the PLL circuit that generates the local sub-carrier used to demodulate the color signal in the television receiver, exhibits a relatively high time constant. Consequently, the PLL circuit is unable to follow relatively brief burst signal phase perturbations, such as those phase inversions that occur every two- or every four-out-of-twenty lines. But, since the APC circuit of the consumer analog video recorder exhibits a low time constant, such APC circuit is able to follow these phase inversions, which are interpreted as phase errors and are used by the video recorder to correct such non-existent errors. Hence, the inherently rapid response time of the video recorder APC circuit results in the recording of deteriorated video signals.
However, when color stripe processing is used to record the video signal of pre-recorded video tapes, such as pre-recorded tapes that are commercially available for sale or rental, the rapid response time of the APC circuit in the playback circuit enables the video recorder to follow and "correct" for such phase inversions. Consequently, when a pre-recorded video tape having color stripe processed video signals recorded thereon is reproduced, the resultant video picture often exhibits undesired defects.